This invention relates generally to solar energy systems. Specifically, the present invention relates to solar energy systems in which a heat transferring liquid, such as water, circulates through a storage tank and a solar collector while the system is active, but drains back from the solar collector to the storage tank after the system becomes inactive.
The "drain-back" feature removes a risk of damage to the solar collector which may occur when the solar collector is exposed to freezing temperatures. Additionally, drain-back prevents convectional circulation of the liquid which may occur when the collector is located above the storage tank, and the collector exhibits a temperature lower than that of the storage tank.
Prior solar energy systems have incorporated drain-back features. However, conventional techniques for draining collectors tend to result in solar energy systems which are less reliable, less efficient, and less economical than desired. For example, some solar energy systems permit drain-back to occur the instant a heat transferring liquid stops moving through the collector. Each time the system initiates circulation of the liquid it must refill the collectors with liquid. Collectors are typically located above a storage tank. Thus, a pump consumes a great amount of time lifting the liquid from a storage tank to the collector in order to refill the collector. A higher energy cost and reduced pump life is associated with operating the pump for such longer periods of time.
Further, solar energy systems which permit drain-back the instant the heat transferring liquid stops moving through the collector demonstrate a reduced efficiency. Such systems typically resume circulation of the liquid when the collector heats to a temperature greater than the temperature of the storage tank. Since no liquid resides in the collector as the collector heats, no liquid heats along with the collector. Thus, such systems lose the opportunity to transfer heated liquid into the storage tank when they initiate circulation of the liquid.
In many prior solar energy systems the pump continuously runs so long as the collector temperature is greater than the temperature of the storage tank. Other systems continuously run the pump as long as a small temperature rise, such as 3 degrees, can be exhibited between liquid leaving the storage tank and liquid entering the storage tank. Either approach forces the pump to operate many hours every day and consume a greater amount of power. Resultingly, a high energy cost associates with operating such systems, and a pump motor experiences a shorter operating life.